Actually, before the Burning Crusade -- or more specifically, daily quests -- came along, I generally would, or rather, I'd buy the herbs and find a friend to do the combines. Others besides me took it one step further and started an alt character specifically to have someone to gather herbs and make potions for them. But even without herbalism, if you watched the AH and herb prices for a bit, you could buy herbs when they were a bit on the cheap side and get yourself a stockpile of cheap elixirs, potions, and flasks.
Ever since daily quests, however, I've been a lot more lazy. I'll find myself at the Auction House less than an hour before raid time praying that someone has the potions I need for sale at a less than soul-gouging prices. I'll buy them at the soul-gouging prices anyway, of course, because I need them, and I have the extra money from dailies, but I wonder if I should go back to the old way of getting the herbs and stockpiling. It'd lead to fewer last minute buying sprees, at least.
So how do you do your consumables? Do you do the last minute buying spree, do you watch for good deals, or do you just do the herbalism and/or alchemy yourself?